Method of controlling crystal growth



United States Pate t Q C? materialsby conditioning and heat treating the material.

The instant application is a continuation-in-part of my copending patent application Serial No. 570,011, filed March 7, 1956, now abandoned. i 7

It is Well recognized in the art that the grain or crystal size or many crystalline materials is highly important to the use towhich the material is to be put; Appertaining to metals utilized for structural purposes, it is .almost a necessity that the metal have as small a grain size as possible. of one or a few large grains is highly desirable for technological study, such .as of the corrosion rates, or magnetic" phenomena and of the physical properties of metals. Oriented, large grained material has found wide acceptance in the fabrication of transformer cores.

Single crystals of germanium and silicon are being used as rectifiers, as transistors, and, in the case of'silicon, in solar batteries. In relation to non-metallic materials, largecrystals of non-metals such as calcium fluoride are much prized as substitutes for glass in certain optical On the other hand, a metallic body consisting systems. Single, large grains or crystals of other non '35 cal property of sodium potassium tartrate' and the radiation detection property ofthe tungstates of calcium and I metals have valuable properties, such as the piezoelectricadmium.

By crystallographic phase transformation or change used"hereina fter is meant any substantial change in crystal structure, such face-centered cubic crystal to a body-centered cubic crysas an allotropic change froina tal, or a change from a solid,'crystalline state to a liquid state.

Certain substances "exhibit allotro'pic' changes which are not considered substantial as=iri-'tlie case 'of cobalt wherein there is a changefrom' a hexagonal closepacked crystal structure toa face-centered cubic crystal structure upon heating to about change involves" only a difference in the sequence of stacking the' close-packed planes" in the c'r'ystal'r 'This type 'of change is notconsidered a crystallographic phase transformationfor the purpose of the presentinvention.

The artof growing old. One'method ingeneral u tion or melt aridcarefrilly controlling the condition of growthby' controlling thethermal g'ra'dients and/or" the disposition of seed crystals. Y 4

This method is satisfactory for materials composed large or single crystals is relatively se includes seeding a" sow-"i1 of crystals which'exist' in only one crystallographici phas'e"' between their freezing point andthe minimum teinpera ture to which the crystals are subjected; newer/sea will not work with materials such as iron,"which solidify in one crystalline form and then'uponfurthercooling change to another changes during cooling seem to prevent the atta'inme'nt 7 of a single crystal at temperatures below about 900 C and the result is a mass of many'smallgrains. "Even with materials having crystals which solidifyand remain in a single allotropic-form, regulating the crystal" size by controlling the heat gradient is 'timeconsurnin'g" and ditficult because it necessitates a high degree of accuracy in the temperature control.

form. With iron, grain "or crystal s For materials composed of crystals which change their crystallographic phase during cooling} another method isoften used to yield large crystals or grams; This methodconsists of introducing energy in some form, e.g. -ma'-- chiming-rolling, -etcL,-into the material and maintaining it at a temperature just below the lowest crystallographic phase transformation temperature for an extended period of time. .Undentheseconditions it hasbeen found that a few grains grow at the expense-cfothers. This; howevergis aslow procss requiringcareful temperature control" over a long time. Furthermore,- this method producescrystals of a certain limited mam--- muni size,"i-.e. they do not increase insize upon further 'ofequal strength with heat-treating, being apparently all respect to growing larger. 1 on the other hand, 'conditions under which an article is fabricated or used-mayapproximate the method described immediately above and produceacoarse or-large grain structure in a material in which such a form is undesirable, e.g. structural metalsr For example, ironcarbon alloys low in-carbon (012% carbon or-less) are subject to grain growth attemperatures between 500-C. and 850 C. Such grain growth-is hastened by repeatedstrains. Articles composed of such alloys often fail due to-the spontaneous coarsening-or growth of their crystals under the influence of heatand strain impartedduring fabrication and/or use, eg welded chain,-furnace tie rods, etc. In like manner, high temperature alloys used in modern day jet aircraft and combustionsysterris have been known to :fail due to spontaneous gram coarsening; t t I It is therefore an object of the present invention to provide a novelmethod'of controlling the grain size'of a crystalline metallic or non-metallic material involving a minimum of time and accuracy. I

Another object of the invention is to provide a method of the character described which maybe usedwit-h crystalline materials which go through crystallographic phase changes during coolingfrom .their: freezing point as well as with crystalline"-materialswhich-do not' exhibit a change in crystallographic phase during cooling;-

Still another object. of the invention is to provide a method which may be used for growing grainsor crystals in a crystalline material of-adesired size and shape;

A- further object-* of the :invention is to provide a method of the type disclosed which is as-equally applicable to preventing the growth-of large crystals-ma fine,- polycrystalline material as'itis'to 'increasing the size of the crystals.

Numerous other objects and advantages of the invenas it is better understood from the tion will be apparent following description, which,-takenin connection with the mentthereoh- I o I have made the surprising discovery that when a fine, polycrystalline primary accompanying drawings, discloses apreferred embodi time or bothstrain-and heat, -the portion ofa the primary material beneath-the coating-"of -secondary substance re-" mains in its original, polycrystalline-form,- .whereas' anyportion of the primary materialnot having ia' coating thereover develops large grains or crystals.

The following example ,is only for I the, purpose of and is not-to be-construed as a scribing the'invention limitation thereon. 1

Example A billet of low carbon (0.8% carbon) steel with a cross section of to form the primary material'test "specimen 2%'-'-'=x 1% a n d N t-1 material is coated over all or part of its surface withalayer-of a--so-called secondary substance and thereafter subjectedto heat -overatperiod of 2% inches by- 2% incheswas cut with a power hack saw normal to its axis to form a slab" 3 X 9 After cutting, the test specimen was etched deeply in aqua regia to remove the strained layer introduced by the cutting operation. The specimen was then coated over about /3 of its surface with a thin electrodeposit of nickel as a secondary substance. The coated specimen had the typical appearance of so-called gray nicke The test specimen was then heated to 850 C. over a period of approximately 1 /2 hours in a high temperature, vacuum furnace. A vacuum of about 75 microns of mercury was maintained during heat treatment. The specimen was maintained at 850 C. for 29 hours and was then cooled to room temperature over a period of 4 hours by cutting off the power to the furnace. The nickel coating was stripped off and the specimen was etched in aqua regia and nital in order to bring out the grain structure. A very large grain of steel grew in the uncoated portion of the specimen and penetrated a short distance beneath the nickel coating. Beneath the nickel coating and beyond the boundary of the large grain, the steel remained in its original fine, polycrystalline form showing that no crystal growth occurred in this portion of the specimen.

The uncoated steel slab Was then reinserted in the furnace and was maintained at 850 C. for an additional 4 days. The large grain increased appreciably in size.

Any crystalline material metallic or non-metallic that is heat stable and will not decompose readily upon heating may be used as the primary material in the present invention. Operable metallic materials are iron, aluminum, nickel, copper, cobalt, titanium, gold, etc.; and alloys thereof such as the steel disclosed in the example above. Examples of operable non-metallic materials are inorganic crystalline materials such as alkali metal halides, tungstates of calcium and cadmium, sodium potassium tartrate, sodium nitrate, silver chloride, calcium fluoride, etc.; and organic crystalline materials such as stilbene, anthracene, etc.

The choice of the secondary or coating substances depends primarily upon its properties and its effect upon the properties (other than crystal growth) of the primary material. The secondary substance should have a strong adherence to the primary material and should be capable of being readily applied thereto such as by electroplating, vapor deposition, liquid coating, decomposition of a com"- pound of the secondary substance or by any means where by an unbroken continuous adherent coating of control lable thickness results. It is advisable to use a secondarysubstance which has a low solubility in or slow rate of solution into the primary material so that the coating will not be removed in whole or in part by dissolution into the body of the primary material, or change the composition and properties of the primary material by dissolving therein.

The coating or secondary substance may be composed of the same material as is the primary material, or it may be a different material provided it meets the requirements set forth above. However, it is necessary that the secondary substance be applied as a coating to the primary material and that an interface exists between the primary and secondary materials.

The thickness .of the coating of secondary substance may be varied over relatively wide limits. It is neces sary only that the thickness be suflicient to form an unbroken, continuous covering over the portion of the primary material inwhich graingrowth is to be inhibited. Thicknesses greater than this minimum requirement may be used but offer no special advantages. Using either nickel or tin as the secondary substance and a low carbon steel of the type disclosed in the example as the primary material, coating thicknesses in the range of 0.001 to 0.02 inch effectivelyinhibited grain growth in the steel.

It is usual and preferable to start with'a primary material having a fine, polycrystalline form. Starting with such a material, it is possible to cause the grains or crystals to grow in any desired direction and desired size; or it is possible to inhibit grain growth and maintain the original, polycrystalline form. Primary materials composed of a number of large crystals may also be treated by the method of the instant invention to increase the size of these large crystals. However, the present invention functions either to inhibit grain growth or to increase the size of crystals. If the starting crystals are already larger than desired, any of the means well known to the prior art may be used to reduce the size of these large crystals before subjecting the material to the steps of the instant invention.

The temperatures operable in the present method will vary depending upon the nature of the primary material; and also the temperatures may be varied within a relatively wide range even when using a given primary material. In the broadest aspect of the invention, it is nec essary only that the temperature be elevated, i.e. above room temperature (20 C.), and that this elevated temperature be sufliciently high to cause a change in crystal size in the particular primary material within a reasonable time. Since it is necessary that the primary material remain solid throughout the process, the temperature must always be maintained below the melting point and decomposition point of the primary material.

In the interest of time, equipment and operating economy, the practicable operating temperatures fall into a narrower range than specified broadly above. These preferred minimum and maximum temperatures are between, respectively, the so-called refining or recrystallization temperature, i.e. that temperature at which a large or coarse grain structure is changed to a small, fine grain structure under the influence of heat alone, of the particular primary material employed and the temperature at which a crystallographic phase transformation (defined hereinbefore) takes place in this particular primary material. Using iron as an example, the preferred range of operating temperatures is between about 500 C., the recrystallization temperature of iron, and about 900 C., the temperature at which an allotropic change takes place in iron from a body-centered cubic lattice existing below 900 C. to a face-centered cubic lattice existing above 900 C.

Under certain circumstances it may be advantageous to carry out the heat treatment at temperatures below the melting point of the secondary substance, such as when the secondary substance, in the liquid phase, does not wet the surface of the primary material, e.g. lead on iron. Under these conditions, the molten, secondary substance will become discontinuous exposing undesired portions of the surface of the primary material. However, with other secondary substances which do wet the primary material, e.g. tin on iron, this problem does not exist since such a coating substance will remain continuous whether molten or solid. Therefore, it is a simple matter to preserve a continuous coating thereover during treatment by selecting a coating substance either that remains solid at the treatment temperature or is sufiiciently compatible with the primary material to wet the surface thereof when molten. Obviously, however, it is necessary that the temperatures to which the secondary substance is subjected. be below its boiling point and decomposition point.

The time interval during which the heat treatment is carried out depends upon a number of independent factors, e.g. the type of primary material, the initial grain or crystal size, the final grain size desired and the temperature of the heat treatment. In general, however, it can be stated that the higher the heat treating temperature, the less time will be required.

The removal of any strained surface layer from the primary material prior to the heat treatment thereof is essential and critical to the success of the instant invention. I have found that the growth of large crystals will not occur if the surface of the primary material wherein layer is most readily accomplished by etching deeply the strained surface with a strong acid such as aqua regia,

hydrochloric, sulfuric ;or nitric acid. 'Another method ofdeep etching and thereby removing the strained layer is by electrolytic etching in the presence of 'a liquidetchant. 1

To hasten crystal growth and lessen the time of heat treatment to produce a desired crystal size, energy may be introduced into the body or interior of the primary material before treatment Such energy may be introduced in the form of mechanical strain, for example that produced by compression, elongation, torsion, etc. Very often the primary material has had sufficient strain imparted to it during fabrication, making further mechanical working unnecessary prior to the crystal growing operation. Other methods of introducing energy or strain into the material are also effective, for example compressing the material or bombarding it with high energy particles such as neutrons or protons. This step of introducing energyinto' the primary material, although it improves somewhat the efliciency of the method of the present invention, is not necessary thereto. It must be understood that the strain produced by the above methods must be confined to the interior of the primary material; and any such strain induced on the surface of the primary material must be removed prior to the heat treatment for the reasons set forth hereinbefore.

In general, the primary and secondary materials, when subjected to the preferred elevated temperatures, are adversely affected by the presence of an appreciable amount of oxygen, such as the causation of rapid oxidative corrosion of iron. To prevent the occurrence of such adverse effects, it is advantageous and preferred to carry out the heat treatment in an inert atmosphere, e.g. argon, or in a reducing atmosphere, e.g. Wet or dry hydrogen gas, or under reduced pressure, i.e. less than 500' microns of mercury.

Removal of the coating of secondary substance can be accomplished by any appropriate means known in the art. For example, the coating may be removed by chemical dissolution (etching), machining, abrading, sandblastin etc.

While not wishing to be bound by any particular theory, examination of test specimens indicates that grain growth originates on the uncoated surface of the primary material and proceeds into the body of the material and toward the coated portion of the material. From this observation, it appears that the crystal or grain growth can be started at any desired place on the surface of the primary material merely by leaving that place uncoated and coating the remainder of the material prior to heat treatment. Since the crystal growth proceeds throughout the uncoated portion and toward the coated portion, the direction of growth of the crystal can be controlled by removing the coating of secondary substance in a predetermined pattern and re-heat treating the material. Unlike crystal size obtained with the prior art methods, in the instant method there appears to be no lessening in growth potential of the large crystal with repeated coating removals and heat treatments. Therefore, the invention provides a method of growing crystals having substantially any desired size or shape.

Further, since grain growth appears to be initiated on the uncoated surface of the primary material, it is also possible to inhibit if not prevent undesired grain growth in a primary material, for example in those materials subject to grain growth under the conditions of use. This is accomplished merely by coating the entire surface of the primary material with an appropriate secondary material.

As a corollary to the above possibilities, the instant method can be used to produce a crystal or grain of the desired size and shape and then, by completely covering the primary material -t'he secondary substance/this shape and size canbe maintained. Such aprocedure will inhibit further grain growth in the primarymaterial even though "it is thereafter subjected to influences tending to increase grain size. 7 p t The instant methodis adaptable to a continuous process whrein'all the'stepscan he carried" out without interruption. By' causing the secondary ma'terialto be stripped backfr'om'. the advancing edge of 'the growing crystal at the same rate as the growth of the crystal, the steps of cooling and reheating and the cessation of "crystal growth caused therebyca'n be eliminated. I

It is apparent also that largecry'stals having a particular orientation can be obtained by'selecting from the plurality of initial crystals or partially grown crystals in thev primary material a crystal' having. a certainforientation. Thereafter, by arranging the coating of secondary substance in the proper pattern, the particular crystal or grain selected having the desired orientation can be caused to grow while inhibiting the growth of the other grains in the specimen.

It is though that, the invention and many of its attendant advantages be understood from the foregoing description, and it will be apparentthat changes may be made in the steps of the method described and their order of accomplishment without departing from the spirit and scope of the invention or sacrificing all of its material advantages, the form herebefore described being merely a preferred embodiment thereof.

I claim:

1. A method of controlling crystal size comprising providing a polycrystalline primary material composed of crystals no larger than the crystal size finally desired, etching the surface. of said primary material to remove any strained layer from said surface material whereby. said primary material is composed of crystals that normally increase in size under the influence of heat alone, applying a continuous, adherent, nonporous, nonpenetrating coating of a secondary substance over at least a portion of the etched surface of said primary material, said secondary substance exhibiting no tendency toward thermal decomposition at temperatures up to its boiling point and having at most only slow solubility in said primary material at temperatures up to the crystallographic phase transformation temperature of said primary material whereby said secondary substance remains on the surface of said primary material as a coating at elevated temperatures with a definite interface existing between said primary material and said coating of secondary substance, heating the thus coated primary material to a temperature between its recrystallization temperature and its crystallographic phase transformation temperature, and maintaining the coated primary material at said temperature for a period of time whereby a large crystal of said primary material is formed in any uncoated portion of said primary material and crystal growth is inhibited in said primary material coated by said secondary substance.

2. The method set forth in claim 1 wherein only a portion of said polycrystalline primary material is coated with said secondary substance.

3. The method set forth in claim 1 wherein said secondary substance is the same as said primary material.

4. The method set forth in claim 1 wherein said secondary substance is different from said primary material.

5. The method set forth in claim 1 wherein the atmosphere surrounding the primary material during heating contains no more than a trace of oxygen.

6. The method set forth in claim 5 wherein the atmosphere surrounding the primary material during heating is maintained at a reduced pressure of less than 500 microns of mercury. I

7. A method of increasing the size of crystals in steel comprising providing a polycrystalline steel element composed of crystals not larger than the crystal size finally desired, etching the surface of said element to remove any strained layer from said surface whereby said element is composed of crystals that normally increase in size under the influence of heat alone, applying a continuous, adherent, nonporous, nonpenetrating, metallic coating over a portion of said etched surface, said coating exhibiting no tendency toward thermal decomposition at temperatures up to its boiling point and having at most only slow solubility in said element at temperatures up to the melting point of said element whereby said coating remains on the surface of said element at elevated temperatures with a definite interface existing between said element and said coating, heating the thus coated element to a temperature below the melting point and decomposi tion point of said element and below the boiling point-and decomposition point of said coating, maintaining the coated element at said temperature for a period of time to form an enlarged crystal in the uncoated portion of said element, removing said coating from the surface of said element adjacent said enlarged crystal to expose an additional portion of said element, and continuing to heat said element at a temperature below the melting point and decomposition point of said element and below the boiling point and decomposition point of said coating to cause said crystal to enlarge further by'growing into said additionally exposed portion of element.

8. The method set forth in claim 7 in which the metallic coating is removed in a predetermined pattern whereby said crystal by growing into said additionally exposed portion of said steel element assumes a predetermined shape.

9. The method set forth in claim 1 wherein each of said primary material and said secondary substance is a metal. 7

10. The method set forth in claim 1 wherein said primary material is steel;

11. The method set forth in claim 1 wherein said primary material is steel and said secondary substance is nickel.

12. A method of controlling crystal size comprising providing a steel element composed of a plurality of crystals no larger than the crystal size finally desired, etching the surface of said element to remove any strained layer from said surface whereby said element is composed of crystals that normally increase in size under the influence of heat alone, electrodepositing a continuous adherent non-porous, non-penetrating coating of nickel over at least a portion of said etched surface, said nickel remaining on the surface of said element as a coating at elevated temperatures with a definite interface existing between said element and said nickel, heating the thus coated element to a temperature between its recrystallization temperature and its crystallographic phase transformation temperature, and maintaining the coated element at said temperature for a period of time whereby a large crystal of said steel is formed in any uncoated portion thereof and crystal growth is inhibited in the portion of said element coated by said nickel.

13. A method of controlling crystal size comprising providing a steel element composed of a plurality of crystals no larger than the crystal size finally desired, etching the surface of said element deeply with aqua regia to remove any strained layer from said surface whereby said element is composed of crystals that normally increase in size under the influence of heat alone, electrodepositing a continuous adherent non-porous, nonpenetrating coating of nickel over at least aportion of the etched surface, said nickel remaining on said surface as a coating at elevated temperatures with a defim'te interface existing between said element andsaid nickel, heating the thus coated element to a temperature of 850 C., and maintaining said coated element at said temperature under vacuum for 29 hours whereby a large crystal of steel is formed in any uncoated portion of said element and crystal growth is inhibited in the portion of said element coated by said nickel.

References Cited in the file of this patent UNITED STATES PATENTS UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent Noe 3 O63,816 November 13 1962 Henry Leidheisem Jr.

It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 2,, line 69, for "0.8%" read 0,08%

Signed and sealed this 16th day of July 1963;,

(SEAL) Attest:

ERNEST w. SWIDER DAVID LADD Attesting Officer Commissioner of Patents 

1. A METHOD OF CONTROLLING CRYSTAL SIZE COMPRISING PROVIDING A POLYCRYSTALLINE PRIMARY MATERIAL COMPOSED OF CRYSTALS NO LARGER THAN THE CRYSTAL SIZE FINALLY DESIRED, ETCHING THE SURFACE OF SAID PRIMARY MATERIAL TO REMOVE ANY STRAINED LAYER FROM SAID SURFACE MATERIAL WHEREBY SAID PRIMARY MATERIAL IS COMPOSED OF CRYSTALS THAT NORMALLY INCREASE IN SIZE UNDER THE INFLUENCE OF HEAT ALONE, APPLYING A CONTINUOUS, ADHERENT, NONPOROUS, NONPENETRATING COATING OF A SECONDARY SUBSTANCE OVER AT LEAST A PORTION OF THE ETCHED SURFACE OF SAID PRIMARY MATERIAL, SAID SECONDARY SUBSTNACE EXHIBITING NO TENDENCY TOWARD THERMAL DECOMPOSITION AT TEMPERATURES UP TO ITS BOILING POINT AND HAVING AT MOST ONLY SLOW SOLUBILITY IN SAID PRIMARY MATERIAL AT TEMPERATURES UP TO THE CRYSTALLOGRAPHIC PHASE TRANSFORMATION TEMPERATURE OF SAID PRIMARY MATERIAL WHERBY SAID SECONDARY SUBSTANCE REMAINS ON THE SURFACE OF SAID PRIMARY MATERIAL AS A COATING AT ELEVATED TEMPERATURES WITH A DEFINITE INTERFACE EXISTING BETWEEN SAID PRIMARY MATERIAL AND SAID COATING OF SECONDARY SUBSTANCE, HEATING THE THUS COATED PRIMARY MATEIAL TO A TEMPERATURE BETWEEN ITS RECRYSTALLIZATION TEMPERATURE AND ITS CRYSTALLOGRAPHIC PHASE TRANSFORMATION TEMPERATURE, AND MAINTAINING THE COATED PRIMARY MATEIAL AT SAID TEMPERATURE FOR A PERIOD OF TIME WHEREBY A LARGE CRYSTAL OF SAID PRIMARY MATERIAL IS FORMED IN ANY UNCOATED PORTION OF SAID PRIMARY MATERIAL AND CRYSTAL GROWTH IS INHIBITED IN SAI DPRIMARY MATERIAL COATED BY SAID SECONDARY SUBSTANCE. 